1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device comprising an SOI (silicon on insulator or semiconductor on insulator) substrate in which a device isolation region is formed.
2. Description of Related Art
SOI substrates have conventionally been used for manufacturing semiconductor devices. On a surface silicon layer which comprises the SOI substrate, a device isolation film is formed to penetrate the surface silicon layer for isolation of the semiconductor devices.
For example, Japanese Unexamined Patent Publication No. HEI 9(1997)-298195 discloses a semiconductor device as shown in FIG. 4(g) utilizing the SOI substrate.
The semiconductor device comprises an SOI substrate 20 including a silicon substrate 21, a buried insulating film 22 and a surface silicon layer 23. Active regions for an N-channel transistor and a P-channel transistor are formed in the surface silicon layer 23 and electrically isolated from each other by a device isolation region. Source/drain regions for the N-channel transistor and the P-channel transistor are also formed in the active regions. Gate electrodes are formed on the surface silicon layer via a gate insulating film, and a silicon nitride film and an interlayer insulating film are formed to cover the gate insulating film, the gate electrode and the device isolation region. Through holes down to the source/drain regions are formed in the silicon nitride film and the interlayer insulating film so that connection with upper wirings can be provided.
Such a semiconductor device is manufactured in the following manner.
As shown in FIG. 3(a), a silicon nitride film 24 is formed on the surface silicon layer 23 on the SOI substrate 20, and then the silicon nitride film 24 is selectively removed from a region for device isolation. Using the silicon nitride film 24 as a mask, the surface silicon layer 23 is thermally oxidized to obtain a first LOCOS oxide film 25.
Then, as shown in FIG. 3(b), the first LOCOS oxide film 25 is selectively removed to form a groove 26 in the surface silicon layer 23.
Then, as shown in FIG. 3(c), a silicon nitride film 27 is selectively formed on the sidewalls of the groove 26 and the thermal oxidization is performed again. The surface silicon layer 23 at the bottom of the groove 26 is partially oxidized to form a second LOCOS oxide film, i.e., a device isolation region 28. The thermal oxidization completely oxidizes the surface silicon layer 23 at the bottom of the groove 26, so that the bottom of the device isolation region 28 contacts the top surface of the buried insulating film 22. The device isolation region 28 electrically divides the surface silicon layer 23 into two active regions. Thereafter, the silicon nitride film 24 used as the mask is selectively removed to form a gate insulating film 29 over the active regions.
Next, as shown in FIG. 3(e), a polycrystalline silicon film containing P-type or N-type impurities is formed on the gate insulating film 29 and patterned into gate electrodes 30. Then, as shown in FIG. 3(f), ion implantation to the active regions is carried out to form source/drain regions 31 in each of the active regions, and a silicon nitride film 32 is formed over the entire surface of the SOI substrate.
Then, as shown in FIG. 3(g), an interlayer insulating film 33 and through holes 34 are formed so that connection with upper wirings can be obtained.
As described above, the semiconductor device according to the prior art forms the active regions that are electrically isolated from each other by performing two LOCOS oxidization steps on the surface silicon layer 23.
The first LOCOS oxidization is performed to reduce the thickness of the surface silicon layer 23 at the bottom of the device isolation region 28 so that the height of silicon island regions surface made of the surface silicon layer 23 and that of the device isolation region 28 surface to be formed later can be adjusted to be flat. Further, through the first LOCOS oxidization, the edges of the silicon islands are formed in a round shape to prevent the generation of parasitic MOS.
The second LOCOS oxidization is carried out to form the silicon nitride film 27 on the sidewalls of the groove 26 as an oxidization resistive mask so as to prevent the formation of bird's beak. Further, the oxidization is intended to relax stress by performing it at high temperature (up to 1100.degree. C.).
In the above manufacturing method of the semiconductor device, the purposes of the first and second LOCOS oxidization steps are described, but the thicknesses of the first and second LOCOS oxide films are not mentioned.
The relaxation of the stress and the thickness of the second LOCOS oxide film are significantly related. A thick second LOCOS oxide film will increase the stress applied around it, and eventually induce crystal defects in the surface silicon layer. This may cause leakage of the obtained transistors.